Mixed-valence tetra- and hexanuclear manganese complexes from the flexibility of pyridine-containing β-diketone ligands

The reactions of [Mn₃O(O₂CCCl₃) ₆(H₂O)₃] with 1-phenyl-3-(2-pyridyl)propane-1, 3-dione (HL¹) and 1-(2-pyridly)-3-(p-tolyl)propane-1,3-dione (HL ²) in CH₂Cl₂ afford the mixed-valence Mn ^II₂Mn^III₂ tetranuclear complexes [Mn₄O-(O₂CCCl₃)₆(L¹) ₂] (1) and [Mn₄O(O₂CCCl₃) ₆L₂²] (2), respectively. Similar reactions employing [Mn₃O(O₂CPh)₆(H₂O)(py) ₂] with HL¹ and HL² give the Mn ^II₃Mn^III₃ hexanuclear complexes [Mn₆O₂(O₂CPh)₈(L¹) ₃] (3) and [Mn₆O₂(O₂CPh) ₈L₃²] (4), respectively. Complexes 1·2CH₂Cl₂, 2·2CH₂Cl ₂·H₂O, 3·1.5CH₂Cl ₂·Et₂O·H₂O, and 4·2CH ₂Cl₂ crystallize in the triclinic space group P1, monoclinic space group P2₁/c, monoclinic space group P2 ₁/n, and monoclinic space group P2₁/n, respectively. Complexes 1 and 2 consist of a trapped-valence tetranuclear core of [Mn ^II₂Mn^III₂(μ₄-0)] ⁸⁺, and complexes 3 and 4 represent a new structural type, possessing a [Mn^II₃Mn^III₃(μ₄-O) ₂]¹¹⁺ core. The magnetic data indicate that complexes 3 and 4 have a ground-state spin value of S = 7/2 with significant magnetoanisotropy as gauged by the D values of -0.51 cm^-1 and -0.46 cm^-1, respectively, and frequency-dependent out-of-phase signals in alternating current magnetic susceptibility studies indicate their superparamagnetic behavior. In contrast, complexes 1 and 2 are low-spin molecules with an S = 1 ground state. Single-molecule magnetism behavior confirmed for 3 the presence of sweep-rate and temperature-dependent hysteresis loops in single-crystal M versus H studies at temperatures down to 40 mK.